kvserver: mvcc first split key should use local max when split key is lower than local max

[kvoli]

Is your feature request related to a problem? Please describe.
We added a check to return no split key in MVCCFirstSplitKey when the range's start key is less than keys.LocalMax #104082.

This check means the function will not return a split key for range1, even though we may wish to split meta2 for load. In practice this probably isn't very common, as meta2 would be split out from size based splits.

The solution is to mirror MVCCFindSplitKey which bumps the start key to keys.LocalMax if not at or higher already.

cockroach/pkg/storage/mvcc.go

Lines 5864 to 5866 in 6235182

	if key.Less(roachpb.RKey(keys.LocalMax)) {
	key = roachpb.RKey(keys.LocalMax)
	}

Jira issue: CRDB-28819